Conventionally, a protection element such as a bimetal and a PTC is built in a motor housing for protecting the motor from being burned. When a motor abnormally generates heat, an electric circuit is intercepted by this protection element and the passage of electric current through the motor is stopped.
However, when the above-mentioned protection element is arranged near the motor, the size of the motor is enlarged to increase the size of an entire device. Therefore, in a motor controller described in JP-H11-164472-A, the above-mentioned protection element is not provided but a control unit for driving and controlling the motor calculates the estimated temperature of the motor on the basis of the magnitude of voltage applied to the motor, a voltage applying time, and the last estimated temperature. In the motor controller described in JP-H11-164472-A, when a calculated estimated temperature becomes not less than a predetermined overheating temperature, driving of the motor is stopped and a stopping state is further maintained until the estimated temperature reaches an overheating protection release temperature.
By the way, since a lot of electrical components are mounted in a vehicle, there is presented a problem that load applied to a vehicle-mounted battery for supplying electric power to these electrical components increases. Therefore, it is necessary to reduce the power requirements of the electrical components.
However, in the motor controller that calculates the estimated temperature of the motor, even in a state where the motor is stopped and then an ignition switch is turned off to thereby stop generating power by driving an engine, an adequate estimated temperature is required when the motor is again driven and hence the motor controller needs to continuously perform the processing of calculating the estimated temperature. For this reason, a microcomputer needs to continuously perform the processing of calculating the estimated temperature although the motor is stationary. Therefore, there is posed a problem that the microcomputer continues consuming the electric power of the vehicle-mounted battery.